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#### A.1.2 Matrices and Arrays in Oct-Files

Octave supports a number of different array and matrix classes, the majority of which are based on the `Array` class. The exception are the sparse matrix types discussed separately below. There are three basic matrix types:

`Matrix`

  A double precision matrix class defined in `dMatrix.h`

`ComplexMatrix`

  A complex matrix class defined in `CMatrix.h`

`BoolMatrix`

  A boolean matrix class defined in `boolMatrix.h`

These are the basic two-dimensional matrix types of Octave. In addition there are a number of multi-dimensional array types including

`NDArray`

  A double precision array class defined in `dNDArray.h`

`ComplexNDarray`

  A complex array class defined in `CNDArray.h`

`boolNDArray`

  A boolean array class defined in `boolNDArray.h`

`int8NDArray`
`int16NDArray`
`int32NDArray`
`int64NDArray`

  8, 16, 32, and 64-bit signed array classes defined in `int8NDArray.h`, `int16NDArray.h`, etc.

`uint8NDArray`
`uint16NDArray`
`uint32NDArray`
`uint64NDArray`

  8, 16, 32, and 64-bit unsigned array classes defined in `uint8NDArray.h`, `uint16NDArray.h`, etc.

There are several basic ways of constructing matrices or multi-dimensional arrays. Using the class `Matrix` as an example one can

- Create an empty matrix or array with the empty constructor. For example:

      Matrix a;

  This can be used for all matrix and array types.

- Define the dimensions of the matrix or array with a dim_vector which has the same characteristics as the vector returned from `size`. For example:

      dim_vector dv (2, 3);  // 2 rows, 3 columns
      Matrix a (dv);

  This can be used for all matrix and array types.

- Define the number of rows and columns in the matrix. For example:

      Matrix a (2, 2)

  This constructor can only be used with matrix types.

These types all share a number of basic methods and operators. Many bear a resemblance to functions that exist in the interpreter. A selection of useful methods include

Method: T& operator () (octave_idx_type) ¶
Method: T& elem (octave_idx_type) ¶

  The `()` operator or `elem` method allow the values of the matrix or array to be read or set. These methods take a single argument, which is of type `octave_idx_type`, that is the index into the matrix or array. Additionally, the matrix type allows two argument versions of the `()` operator and `elem` method, giving the row and column index of the value to get or set.

Note that these functions do significant error checking and so in some circumstances the user might prefer to access the data of the array or matrix directly through the `fortran_vec` method discussed below.

Method: octave_idx_type numel (void) const ¶

  The total number of elements in the matrix or array.

Method: size_t byte_size (void) const ¶

  The number of bytes used to store the matrix or array.

Method: dim_vector dims (void) const ¶

  The dimensions of the matrix or array in value of type `dim_vector`.

Method: int ndims (void) const ¶

  The number of dimensions of the matrix or array. Matrices are always 2-D, but arrays can be N-dimensional.

Method: void resize (const dim_vector&) ¶
Method: void resize (nrows, ncols) ¶

  A method taking either an argument of type `dim_vector`, or, in the case of a matrix, two arguments of type `octave_idx_type` defining the number of rows and columns in the matrix.

Method: T* fortran_vec (void) ¶

  This method returns a pointer to the underlying data of the matrix or array so that it can be manipulated directly, either within Octave or by an external library.

Operators such as `+`, `-`, or `*` can be used on the majority of the matrix and array types. In addition there are a number of methods that are of interest only for matrices such as `transpose`, `hermitian`, `solve`, etc.

The typical way to extract a matrix or array from the input arguments of `DEFUN_DLD` function is as follows

    #include <octave/oct.h>

    DEFUN_DLD (addtwomatrices, args, , "Add A to B")
    {
      if (args.length () != 2)
        print_usage ();

      NDArray A = args(0).array_value ();
      NDArray B = args(1).array_value ();

      return octave_value (A + B);
    }

To avoid segmentation faults causing Octave to abort, this function explicitly checks that there are sufficient arguments available before accessing these arguments. It then obtains two multi-dimensional arrays of type `NDArray` and adds these together. Note that the `array_value` method is called without using the `is_matrix_type` method. If an error occurs when attempting to extract the value, Octave will print a message and throw an exception. The reason to prefer this coding structure is that the arguments might be a type which is not an `NDArray`, but for which it would make sense to convert them to one. The `array_value` method allows this conversion to be performed transparently when possible. If you need to catch errors like this, and perform some kind of cleanup or other operation, you can catch the `octave_execution_error` exception.

`A + B`, operating on two `NDArray` objects returns an `NDArray`, which is cast to an `octave_value` on the return from the function. An example of the use of this demonstration function is

    addtwomatrices (ones (2, 2), eye (2, 2))
          ⇒  2  1
              1  2

A list of the basic `Matrix` and `Array` types, the methods to extract these from an `octave_value`, and the associated header file is listed below.

Type | Function | Source Code
--- | --- | ---
`RowVector` | `row_vector_value` | `dRowVector.h`
`ComplexRowVector` | `complex_row_vector_value` | `CRowVector.h`
`ColumnVector` | `column_vector_value` | `dColVector.h`
`ComplexColumnVector` | `complex_column_vector_value` | `CColVector.h`
`Matrix` | `matrix_value` | `dMatrix.h`
`ComplexMatrix` | `complex_matrix_value` | `CMatrix.h`
`boolMatrix` | `bool_matrix_value` | `boolMatrix.h`
`charMatrix` | `char_matrix_value` | `chMatrix.h`
`NDArray` | `array_value` | `dNDArray.h`
`ComplexNDArray` | `complex_array_value` | `CNDArray.h`
`boolNDArray` | `bool_array_value` | `boolNDArray.h`
`charNDArray` | `char_array_value` | `charNDArray.h`
`int8NDArray` | `int8_array_value` | `int8NDArray.h`
`int16NDArray` | `int16_array_value` | `int16NDArray.h`
`int32NDArray` | `int32_array_value` | `int32NDArray.h`
`int64NDArray` | `int64_array_value` | `int64NDArray.h`
`uint8NDArray` | `uint8_array_value` | `uint8NDArray.h`
`uint16NDArray` | `uint16_array_value` | `uint16NDArray.h`
`uint32NDArray` | `uint32_array_value` | `uint32NDArray.h`
`uint64NDArray` | `uint64_array_value` | `uint64NDArray.h`

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